The present invention relates generally to mechanisms for protecting mechanical drive components from overloads, and more particularly relates to a shear device coupled between components of an agricultural disc mower that protects the various components of the mower in the event a cutterhead strikes an object and creates an overload condition.
Typical disc cutterbars used in agriculture include an elongated housing containing a train of meshed idler and drive spur gears, or a main power shaft coupled by respective bevel gear sets, for delivering power to respective drive shafts for cutterheads spaced along the length of the cutterbar. The cutterheads each comprise a cutting disc including diametrically opposed cutting blades (though configurations with three or more blades are known) and having a hub coupled to an upper end of a drive shaft, the lower end of the drive shaft carrying a spur gear in the case where a train of meshed spur gears is used for delivering power, and carrying a bevel gear of a given one of the bevel gear sets in the case where a main power shaft is used. In either case, bearings are used to support the various shafts. The cutterheads are rotated at a relatively fast speed making the drive components, such as gears, bearings and shafts, vulnerable to damage in the event that the unit strikes a foreign object. For background information on the structure and operation of some typical disc cutterbars, reference is made to U.S. Pat. No. 4,815,262, issued to E. E. Koch and F. F. Voler, the descriptive portions thereof being incorporated herein in full by reference.
In order to minimize the extent of such possible damage to the drive components, it is known to incorporate a shear device somewhere in the drive of each unit that will xe2x80x9cfailxe2x80x9d upon a predetermined overload being imposed on the device. As used herein with reference to shear devices, the terms xe2x80x9cfailxe2x80x9d or xe2x80x9cfailingxe2x80x9d are intended to cover the actual function of such devices, i.e., shearing, fracturing, breaking and the like. Several different such shear devices and arrangements are shown in U.S. Pat. Nos. 4,999,981, 4,497,161 and 5,715,662.
The ""981 patent shows a shear mechanism that comprises a shaft with a weakened portion created by a cut groove, or break zone 41 (seen, for example, in FIG. 3 thereof) in driven shaft 20. Upon overload, the shaft breaks at zone 41 that is located outside the support bearing such that there is a clean and complete break in the shaft. This structure is intended to eliminate the input of kinetic energy to the cutterhead after failure of the shear mechanism, thereby eliminating damage to the drive system and gearing. While this structure may in fact eliminate the input of further kinetic energy, it does not stop rotation of the cutterhead or prevent the damage that continued rotation would generate.
A somewhat different shear mechanism is disclosed in FIGS. 2 and 3 of the ""161 patent. Cutting disc 3 is connected by a series of shear bolts 26 to the vertical shaft 8. Upon impact of the cutterhead with an obstruction, the shear bolts fail, stopping the input of rotational force to the cutterhead. FIG. 4 shows a slightly different embodiment where a resilient cover plate 28 depresses balls 30 arranged in holes of the disc 3 and fitting into recesses 31 of the disc 27. An overload impact is intended to cause balls 30 to snap out of the recesses 31 so that the direct rotary joint between shaft 8 and cutting disc 3 is interrupted. It is stated that the connection can be reestablished by continuing to rotate disc 3 with respect to the disc 27 so that the balls 30 again snap into the recesses 31. The embodiments set forth in this patent exhibit the same shortcomings as seen in the ""981 patent, i.e., standard shear mechanisms do not stop rotation of the cutterhead, and thus do not prevent additional damage thereby encountered.
The shear mechanisms shown in the ""662 patent each employ shearable splines. In a first embodiment the shear device is in the form of either a collar or clamping member having internal splines received on a splined upper end of the drive shaft and having shearable cylindrical drive lugs engaged with complementary shaped openings provided in an upper surface of a disk hub. Referring more specifically to FIG. 2 thereof, the upper end of drive shaft 26 has a splined section 86. Shear collar 88 establishes a drive connection between shaft 26 and hub 80. The collar 88 includes internal splines 90 engaged with the splined section 86 of shaft 26 just above hub 80. Shearable cylindrical drive lugs 92 project downwardly from the bottom of collar 88 and are received in complementary holes 94 in hub 80. An overload situation causes the lugs 92 to shear and the continuing transfer of rotational power to cease. FIGS. 4 through 6 show another embodiment where shaft 34 has a splined upper end section 110. Instead of a shear collar, a shear device in the form of a cap-like clamping member 114 is used for transferring torque from shaft 34 to hub 80. Clamping member 114 has an annular lower portion 116 provided with interior splines 118 engaged with the splined section 110 of shaft 34. A plurality of shearable lugs 120 extend downwardly from lower portion 116 and are received in complementary shaped cylindrical openings 94 in hub 80, whereby torque is transferred from shaft 34 to hub 80. Again, when an overload occurs, lugs 120 shear, and torque is no longer transmitted. The final embodiment shown in the ""662 patent is shown in FIGS. 7 through 9. Instead of a disk hub 80, a disk hub 127 is used which has a central splined opening 128 disposed in spaced concentric relationship t the splined upper end section 110 of shaft 34. A ring-like shear insert 130 is received on the upper end of the drive shaft 34 and has inner splines 132 engaged with the splined upper end section of the shaft and has outer splines 134 engaged with the splined opening 128 of hub 126. Splines 132 are designed to shear upon overload.
Similar to the devices discussed above, the embodiments of the ""662 patent do not stop the cutterhead from rotating, even after power is cut off by a shear device. The third embodiment shown in this patent exhibits an additional shortcoming in that upon failure of the shearable splines, the broken pieces tend to become temporarily xe2x80x9cjammedxe2x80x9d in among the other parts and components, resulting in even further, though short lived, torque to be transferred, and the resultant additional damage to the cutterheads.
Particularly in its preferred embodiment, the instant invention overcomes the drawbacks and shortcomings of the prior art. A two-piece hub design, with a spring mounted ball and detent as a shear mechanism therebetween will fail with substantially no residual transfer of torque. The use of this unique shear mechanism results in no broken pieces to become xe2x80x9cjammedxe2x80x9d in among the other parts and components, and can be easily repaired by simply realigning the top and bottom hubs so that the spring-loaded ball in the top disc hub fits into the detent in the lower disc hub. Upon failure, the two-piece hub, one of which is driven directly by the drive shaft, separates and the upper disc hub is driven up a specially threaded retaining bolt and separates from the lower hub and drive shaft. This upward movement separates the upper disc hub from the drive train and removes the affected cutting implement from the path of the other cutterheads on the cutterbar. The upper disc hub continues to rotate upward until it reaches a threadless portion of the retaining bolt. There, the upper disc hub is permitted to rotate freely until the absence of drive train inertia causes it to stop.
Clearly, the concept of a shear mechanism is not new, however the use of a spring-mounted ball and detent instead of a pin, lug, or bolt, as well as the utilization of a specially threaded retaining bolt, provides advantages in overcoming the problems and shortcomings of the prior art as discussed above. In order to limit the damage to a cutterbar in an overload situation, two characteristics are pursuedxe2x80x94a quick, clean disengagement of the driven elements, and the prevention of damage to adjacent discs on the cutterbar by rapid removal of the affected disc from the cutting plane. For non-traditional shear mechanisms, attention is directed to U.S. Pat. No. 2,056,785 (rubber), U.S. Pat. No. 3,064,454 (solder, glass, and other fracturable and fusible materials), and U.S. Pat. No. 3,521,464 (plastic).
Accordingly, one object of the present invention is to provide a specially threaded retaining bolt, operating in conjunction with a shear mechanism, in a mechanical drive train for an agricultural cutterbar that will not only stop the transfer of power along the drive train in the event of overload, but also remove the affected disc hub from the path of other cutterheads on the cutterbar.
It is another object of the present invention is to provide a novel shear device between hub components of a cutterhead.
It is a further object of the present invention is to provide a disc cutterbar with multiple cutterheads, each comprising a drive shaft connected to an inner hub which is connected to an outer hub via a shear mechanism. Upon failure of the shear mechanism, the upper hub and blades are rotated to a position above the cutting plane and out of the path of other cutterheads on the cutterbar.
It is yet a further object of this invention to provide an improved disc cutterbar that is relatively durable in construction, inexpensive of manufacture, carefree of maintenance, easy to assemble, simple and effective in use, and less likely than prior art cutterbars to sustain costly damage upon contact with a fixed object.
These and other objects, features and advantages are accomplished according to the instant invention by providing a disc cutterbar having a two-piece mounting hub, one piece rotatably driven and the other supporting a knife for severing standing crop material, with spring-mounted ball and detent devices holding the two pieces members together and forming a shear device therebetween. A specially threaded retaining bolt is associated with the knife-supporting piece whereby, upon failure of said shear device, the knife-supporting piece is rotated out of the cutting plane and away from the operational cutterheads.